Most manufacturers of gas turbine engines are evaluating advanced multi-wall, thin-wall turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. However, cooling schemes for advanced high-thrust aircraft engines are complex, often involving multiple, thin walls and non-planar cooling features. The ceramic cores that define these advanced cooling schemes are conventionally formed by forcing ceramic compound into steel tooling, but core complexity is limited by the capabilities of tooling design/fabrication. Therefore, complex advanced cooling schemes often rely on the assembly of multiple ceramic core pieces after firing. Assembly requires specialized labor and results in core dimensional variability due to mismatch between assembled core components, while the fragile nature of fired cores results in elevated handling scrap, and compromises to the advanced cooling schemes are required to allow for assembly and positioning of the core assembly or multiple core pieces in the subsequent casting.
Some core geometries require the formation of multiple fugitive core inserts to define features that do not operate in common planes, including: (1) multiple skin core segments, (2) trailing edge features (e.g., pedestals and exits), (3) leading edge features (e.g., cross-overs), and (4) features that curve over the length of the airfoil. Forming multiple fugitive inserts and assembling them in a core die presents a similar problem to that created by core assembly. Intimate contact between inserts may not be insured when they are loaded into a core die, either due to dimensional variability in the individual inserts or poor locating schemes in the core die. Subsequent molding of the ceramic core material may result in formation of flash at the union of two fugitive insert segments. While flash is common in ceramic core molding and is removed as part of standard processing, flash around or between fugitive inserts may reside in hidden, internal cavities or as part of intricate features, where inspection and removal is not possible. Any such flash remaining in the fired ceramic core can alter air flow in the cast blade or vane.
U.S. Pat. Nos. 5,295,530 and 5,545,003 describe advanced multi-walled, thin-walled turbine blade or vane designs which include intricate air cooling channels to this end.
In U.S. Pat. No. 5,295,530, a multi-wall core assembly is made by coating a first thin wall ceramic core with wax or plastic, a second similar ceramic core is positioned on the first coated ceramic core using temporary locating pins, holes are drilled through the ceramic cores, a locating rod is inserted into each drilled hole and then the second core then is coated with wax or plastic. This sequence is repeated as necessary to build up the multi-wall ceramic core assembly.
This core assembly procedure is quite complex, time consuming and costly as a result of use of the multiple connecting and other rods and drilled holes in the cores to receive the rods. In addition, this core assembly procedure can result in a loss of dimensional accuracy and repeatability of the core assemblies and thus airfoil castings produced using such core assemblies.
U.S. Pat. No. 6,626,230 describes forming multiple fugitive (e.g. wax) thin wall pattern elements as one piece or as individual elements that are joined together by adhesive to form a pattern assembly that is placed in a ceramic core die for molding a one-piece core.
U.S. Pat. No. 7,258,156 describes the use of ceramic cores and refractory metal cores that are used to form trailing edge cooling passage exits or convoluted airfoil cast-in cooling features wherein the cores are removed to define internal cooling features.
Copending application U.S. Ser. No. 13/068,413 filed May 10, 2011, of common assignee herewith, describes a method of making multi-wall ceramic core wherein at least one fugitive core insert is pre-formed and then at least another fugitive core insert is formed in-situ connected to the pre-formed core insert to from complex cores with internal walls that cannot be readily inspected or repaired once the core is formed.